If you have an open tank then I would look around the tank area just to check and see if they jumped out. If so, it looks like you need to lower your water level (surface of the water lower down to prevent them reaching the top of the tank when they jump).
As StarBetta previously mentioned also, it may be the case that they are underneath the gravel (unlikely, but still possible), you may want to dig your hands into your substrate and swish around, if they are stuck unfortunately I can hardly imagine that they are alive.
Also, how hard are you looking? You never know, if you have any decor/plants (live/artificial) try to lift it (of course if you have live plants, don't lift those) and have a real good search.
Finally, are there any other fish in the tank that are larger than the betta? I trust that you wouldn't put a sorority with larger fish that are threatening to bettas so I can't imagine that being the case, thought I'd put it out there.